Low wattage, compact, single-ended, fluorescent lamps of H-shaped tubular design are known and have been used in lighting fixtures. These lamps are also known as "twin tubes" and are available commercially from North American Phillips Lighting Company, Hightown, N.J., Osram of Newburgh, N.Y. and other vendors and are disclosed in detail in the various manufacturer's specification sheets.
These compact, energy saving, fluorescent lamps function in a manner similar to the larger tubular fluorescent lamps which are coated on the inside with a phosphor and are filled with mercury vapor and with a small amount of argon to facilitate starting. They also employ a preheat type of starter switch built into the lamp base, and have an external choke (ballast) in series with one side of the line voltage to limit the current flow when the lamp is operating. The recently available H-shaped, low wattage fluorescent lamps are available in 5, 7, 9 and 13 watts and like the larger, older type, linear lamps, they are temperature sensitive. The 5 and 7 watt H-shaped lamp is able to start at an environmental temperature down to about 0.degree. F. and the 9 watt lamp functions at a temperature down to about 25.degree. F., but these lamps have limited lumen output. The higher lumen output 13 watt H-shaped lamp, however, which has become available recently, either does not start at temperatures below about 32.degree. F. or requires a relatively long time to light fully and its long starting time causes a reduced lamp life and thus these lamps cannot be used in lighting fixtures exposed to such low temperatures. Similarly, a most recent type of fluorescent lamp in wattages of 10, 13, 14, 18, 20, and 26 watts available from North American Phillips is known as "quads" and are clusters of two compact H-shaped lamps in a single base. These lamps are subject to lighting problems below about 32.degree. F. and like any of these compact lamps, such lamps above 7 watts are excluded from use in outdoor lighting fixtures in cold weather climates, in refrigerated rooms and the like. The present invention provides a means to overcome this deficiency.